NL7907407A - INSTRUMENTATION SYSTEM. - Google Patents

Description

- 1 - ï

Instrumentation system.

The invention relates to instrumentation systems for use in industrial processes.

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More particularly, the invention relates to systems in which a measurement signal corresponding to a process condition, for example a differential pressure or the like, is sent from a field location j to a signal receiving location such as an instrumentation | control room. The invention will be described in its preferred embodiment for enhancement in the transmission of signals generated by a force sensor of the signal. vibrating wire type.

! Force sensors of the vibrating wire type are already »| known in the art for years. Recent developments in such sensors have resulted in much improved operation, so that the use of vibrating wire! instrumentation is starting to increase significantly in the process industry. A particularly advantageous instrument of this type is described in copending U.S. Patent Application Serial No. 834,481.

: 20 The use of a vibrating wire sensor as part of a modern instrumentation system requires an electrical signal transmitter for the sensor, in order to develop a suitable signal corresponding to | with the force applied to the vibrating wire. Such a transmitter is described in U.S. Patent 4,118,977. In this transmitter, the electronic circuit for the vibrating wire sensor comprises a | oscillator, which is coupled via a two-wire cable to the vibrating wire to induce vibrations to the resonant frequency of the wire, and frequency to analog converter means for converting the oscillation frequency into a corresponding DC 790 74 07. Λ --2 signal, eg in the range of 4 to 20 ma, suitable; i to be sent to a central station by a second two-wire cable. The electrical circuitry in this embodiment is usually located immediately adjacent the sensor, but may be remote from the sensor by a certain distance which is; bounded by the characteristics of the two-wire cable, which is coupled to the vibrating wire. ;

The transmitter circuit shown in U.S. Patent 4,118,977 further includes additional; signal processing means for scaling the direct current signal to correspond to a predetermined measuring range, and for characterizing the signal to provide a linear relationship with respect to the applied force. Thus, the final DC measurement signal can be used to adapt to the equipment of existing instrumentation systems using corresponding DC sensor signals, control signals, and the like, eg, computer-operated systems as described in copending U.S. Patent Application Serial No. 737,195. Although a known transmitter device as described above works well, improvements are desired in certain important respects. For example, it has proven to be of particular advantage to reduce the amount of electronic equipment required at the transmitter site. Furthermore, it has proved highly desirable to be able to transmit additional information on the same transmission channel between the field station and the exchange; post. Furthermore, experience has shown the need to use a simple and inexpensive means at the transmitter to indicate to field service personnel the actual value of the measurement as it appears to the remote control room. The invention is now directed to the provision of 790 7 407. Λ * - 3 - improvements in this and other respects.

In a preferred embodiment of the invention, the electronic equipment required in the field is substantially reduced by means of an arrangement in which the transmitter produces a rapidly varying alternating signal which is transmitted over the conventional two-wire cable to the control room. The measurement information is represented by the frequency of this alternating signal. The control room is equipped with a suitable signal processing equipment to convert the received frequency signal into a corresponding analog signal, eg a voltage signal of 0-10 volts. In addition, the signal processing equipment provides other necessary functions, such as scaling the signal size to obtain adaptation in a predetermined area, and linearizing the signal.

According to another aspect of the invention, additional signal information may be sent over the two-wire cable carrying the rapidly varying AC signal to serve as an auxiliary function.

This result is obtained by slowly varying the DC level in the two-wire cable in accordance with such additional information. In the preferred embodiment of the invention, such additional information is sent from the control room to the transmitter to indicate the final processed measurement signal developed in the control room in response to the raw frequency-30 signal from the transmitter. This final measurement signal value is available from the transmitter in the form of a DC signal, and thus can be easily displayed by conventional indicia to show the field service personnel the value of the final measurement developed in the control room. Such a visual indication is l -------------___ 790 74 07 - $ - 4 - ----- ---------- - ----- helpful when setting up and calibrating that 1 part of the instrumentation system.

Accordingly, it is an object of the invention to provide an improved instrumentation system in which a transmitter transmits condition measurement signals to another location for process processing and the like. Other objects, aspects, and advantages of the invention will now be explained in part, and in part will become apparent from the following description of a preferred embodiment of the invention with reference to the drawing. In the drawing shows:

Fig. 1 is a perspective view of an instrumentation system according to the invention, FIG. 2 is a simplified diagram of the basic parts of the system of FIG. 1, FIG. 3 is a detailed diagram of the transmitter portion of the pile driving system of FIG. 2, and; Fig. 4 shows a detailed schematic of the control; 20 chamber section of the system of fig. 2-

First of all reference is made to Fig. 1, in which there is one in the field on the left. mounted sensor 10 shown in the form of a vibrating wire type differential pressure measuring instrument, as is commonly used for measuring fluid flow rates. Details of such a sensor are given in the aforementioned co-pending U.S. Serial No. 834,481. Briefly, the instrument includes a taut wire tensioned in accordance with the differential pressure to be sensed, so that the resonant frequency of the wire is a function of the differential pressure. This wire is coupled to an oscillator circuit, which is part of a transmitter unit 12, and arranged to induce physical vibrations of the wire 35 at its resonant frequency.

Transmitter 12 is coupled via a two-wire transmission cable 790 7 4 07 a ψ - 5 - 14 to the signal receiving equipment at a remote central station, e.g. a control room, located at a location about 1.5 km away from the field station . The control room equipment is shown as consisting of a circuit board 16 containing circuits for performing signal processing and other functions to be described. The two-wire cable carries to the control room a rapidly varying signal derived from the transmitter oscillator output signal, so that the frequency of this alternating signal corresponds to the voltage of the vibrating wire, which in turn corresponds to the applied force being measured. The circuit incorporated in panel 16 transforms this raw AC signal into an analog measurement signal in the range of 0 to 10 volts, and is appropriately processed to provide an accurate measure of the differential pressure. This analog signal is directed to still further equipment of the total system, in fig. 1 for the sake of simplicity! 20 explained as a known indication device 18, as described in the related American | Serial No. 809,148.

! According to one aspect of the invention, the two-wire cable 14 carries a DC component of limited mean value, and the transmitter circuit] at field station comprises means for using | this average DC component for powering all operating electrical power for | the oscillator, while simultaneously supplying to the two-wire cable 30 the rapidly varying voltage signal corresponding to the frequency of the wire vibration, | According to a further aspect of the invention is the size of the average component of the 35 DC in the two-wire cable · '14 adjustable at - a relatively low frequency (eg between 0 and 5 Hz), 79074 07 i τ · - 6 - to carry additional information between the two to post. In the preferred embodiment, the circuit located in the control chamber is operable to vary the average DC component in accordance with the value of the final analog measurement signal, ie the scaled and linearized signal derived from the raw measurement signal of the transmitter 12.

Thus, this modified analog signal information is made available to the remote field station, and can, for example, be used to operate a visually perceptible indicator, such as the flow meter, indicated by 20. The easy availability of such information to the field location can assist service personnel in ensuring proper operation of the system. "

In flg. 2 ,. in which the measuring system is shown in more detail, the sensor 10 is symbolically represented by the vibrating wire 30, which is mounted between two supports 32 and 34 and electrically coupled via a transformer 30 with an oscillator 38. In a known manner, this oscillator supplies energy to wire 30 in order to vibrate it at its resonant frequency, corresponding to its tension force.

Two circuits are coupled to the oscillator 38, generally indicated by 40 and 42, which are connected in series with each other, and in series with the two-wire cable 14. At the other end of the two-wire cable, a conductor 14a is connected via a resistor 44 connected to a positive voltage source. The other conductor 14b is connected through a constant voltage source 48, which includes a transistor 50 and an emitter resistor 52, connected back to the negative supply voltage terminal. With this arrangement, a constant current I flows through the two-wire cable, which has a magnitude which is partly determined by the emitter resistor - 52, and which is controllable by the voltage applied 7907407 *.

- 7 - at the base of the transistor 50.;

The series circuit 40 comprises a Zener diode 54 parallel to a transistor 56, the base of which is coupled to the output terminal 58 of the oscillator 38. The oscillator output signal alternatively drives the transistor 56 between its conducting and non-conducting states at a relatively fast frequency e.g.

in the range of 1700-3000 Hz, corresponding to the vibration frequency of the wire 30. When the transistor 10 is not conducting, I flows through the Zener diode 54, which av has a breakthrough voltage of about 3 volts.

When the transistor 56 conducts, - 1 flows through the transistor, which has a saturation voltage of about 0.25 volts. Thus, circuit 40 gives the two-wire cable 14 a rapidly changing AC voltage of about 2.75 volts amplitude.

ί Part of the stream I. through the two-wire cable 1 av flows through the second series circuit 42, which consists of another Zener diode 60. This circuit develops a DC voltage, which is regulated at a constant! size due to the Zener diode characteristic, and which i | serves as a DC supply voltage for the | oscillator 38 and all associated circuit elements i at transmitter 12.

The rapidly varying alternating current developed by the first series circuit 40, as described above, is passed through the two-wire cable 14, and through a coupling capacitor 62 to a signal processing circuit, generally indicated by 64. This circuit performs a number of different functions. when transforming the raw AC signal from transmitter 12 into a suitably conditioned analog measurement signal, compatible with other information and control signals used for the overall instrumentation system.

In particular, the signal processing L —------- includes 790 74 07 * t “8” ............ . Circuit 64 has a frequency-to-analog converter 70 for producing an analog signal corresponding to the frequency of the incoming alternating signal on the two-wire cable 14. The signal processing circuit further includes a signal characterizer 72 operable on the signal data from the transmitter 12 in order to provide a linear relationship between the value of the final measuring signal and the magnitude of the force applied! 10 on the vibrating wire. The signal processing circuit also includes a scale 74 which adjusts the zero and span characteristics of the final measurement signal, in this example specifically about one; voltage signal in the range of 0 to 10 volts. * i

The final analog measurement signal from the signal processing output 56 is supplied to a difference adder 90 together with a DC voltage signal, which is passed through a filter 82 of the emitter resistor 52. The difference output of the adder is coupled to an amplifier 84, which the base of the constant current source transistor 50 floats. Thus, this circuit provides a feedback control which continuously adjusts the current through transistor 50 to a value directly corresponding to the magnitude of the scaled measurement signal at the output of signal processing equipment 64. j

Since the current flowing through transistor 30 is the same as current I through two-wire AV cable 14, this current gives transmitter 12 accurate information for the actual value of the final measurement signal developed from the raw Alternating measurement signal generated by the oscillator 38. This direct current flows through the meter 20 to provide field service personnel with a readily available indication of the actual measurement signal level generated by the instrumentation system for the field service personnel. control room.

Fig. 3 shows a detailed circuit diagram 5 of the transmitter 12. It can be seen that | the secondary winding of the vibrating wire transformer 36 is coupled to the oscillator circuit, generally indicated at 38, which is substantially the same as that described in the aforementioned U.S. Patent 4,118,977. This oscillator comprises a differential amplifier 90 which drives a second amplifier 92, the output of which is coupled via split positive feedback paths to the inputs of the differential amplifier 90. In operation, this oscillator produces a square wave alternating signal at the resonant frequency of the | vibration wire in the sensor 10.

The square wave oscillator signal at the output terminal 58 is directed through an amplifier 96 to a Darlington switched transistor switching circuit, generally designated 98, which includes the S transistors 55 and 56. The latter transistor i, as previously described, is connected in parallel i with a Zener diode 54 in series with the two-wire cable j 14, and serves for applying to this cable j a square wave alternating voltage signal of approximately i 2.75 volt amplitude. Also connected in series with the two-wire cable is a second Zener diode 60, which develops a unidirectional voltage from the DC current in the cable that serves as the supply voltage for the oscillator 38 and all power supply-demanding circuit elements at the transmitter.

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Fig. 4 shows a detailed diagram of the control room electronics for the measuring system.

Thereby, the square wave alternating signal on the incoming two-wire cable 14 is coupled via the capacitor 62 to a pulse signal conditioning circuit 100 comprising a preamplifier 102. The output of this! amplifier is directed to a series-connected | i set of NOR gates 104, 106, 108, 110, which are essentially! ! 5 correspond to those described in the aforementioned: i

U.S. Patent 4,188,977. The outputs j of the last two gates 108 and 110 provide amplified square wave switching control signals A and B, which are 180 ° out of phase with each other, as graphically]

10 shown on the drawing. This switching control I

signals are used to operate the frequency-to-analog converter means which is provided for developing an analog measurement signal from the raw AC signal on the two-wire cable 14.

The square wave switching control signals A and B j are directed to respective switches S1 and 1 S2, which are connected in parallel to series-switched capacitors 112 and 114, respectively, and serve to alternately short-circuit these capacitors to the frequency of the alternating signal on the cable 14.

The top terminal of the top capacitor 112 receives current through a resistor 116 connected to a controlled source for a reference voltage VR. A parallel connected potentiometer 118 and an isolation resistor 120 derive an adjustable small portion of this current to allow zero trim of the instrumentation system, as will be described in more detail.

As set forth in the aforementioned U.S. Pat. No. 4,188,977, the operation of switches S1 and S2 serves to absorb current received from resistor 116 to an extent proportional to the resonant frequency of the switching operation. The excess current received from resistor 116 (ie, the current not absorbed by the capacitor switching circuit 790 74 07 11 -11 or derived by the zero potentiometer 118) becomes directed to one terminal of an operational amplifier 130, and flows through a negative feedback resistor 132 connected around this terminal. 5 stronger. When set to operation, the zero potentiometer 118 is adjusted to derive an amount of current producing an amplifier output voltage zero when a zero differential pressure is applied to the sensor 10. Under these 10 conditions, the vibrating wire 30 has a sensor! the resonant frequency of about 1700 Hz at a zero input force.

With this arrangement, the output of amplifier 130 will be a DC voltage, which is directly proportional to the difference between the actual! resonant frequency of the vibrating wire and its zero input resonant frequency (ff J. This DC voltage j is supplied as input to a second capacitor switching circuit 134, which, like the previously described], has a pair of switches S3 and S4, which are controlled by the switching control signals A and B. This capacitor switching circuit operates a multiplication function for developing a current flux proportional to the product of (1) the previous amplifier output voltage and (2) the frequency of the switching actuation. previous amplifier is proportional to the zero-corrected wire resonant frequency (f-f), the current generated by circuit 134 contains a component proportional to the square of the resonant frequency.

This quadratic transfer function, developed | through the capacitor switching circuit 34, serves to characterize the current signal supplied thereby to provide linearization of the relationship between the current and the applied shield pressure. This current is directed to an input terminal of a second operational amplifier 136, and flows through a negative feedback resistor 138 to a span adjusting network 140, 5 connected to the output of the amplifier. This | Span circuitry includes an adjustment potentiometer 142 and a pair of series-connected resistors 144 j and 146. j!

The input of the second operational amplifier 1036 also receives via a resistor 150, which is parallel I.

is connected to the capacitor switching circuit 134, \ a current which is proportional to the output voltage j of the first operational amplifier 130, i.e. <i proportional to the zero-corrected resonance frequency (Η 1 of the vibrating wire. current serves to further characterize the total current signal applied to the second operational amplifier 136, to refine the linearization of the relationship between the signal and the applied differential pressure.

The output voltage of this operational amplifier 136 is zero when there is a zero differential pressure. is applied to the sensor 12. For finite value inputs, the amplifier output includes two frequency sensitive components, one of which is proportional to the square of the wire resonant frequency and the other proportional to the first power of this resonant frequency. These two components provide j. very close characterization adjustment with the non-linear relationship between the applied force and the resonance frequency of the vibrating wire, j producing output voltage 160 which varies substantially linearly with respect to the differential pressure applied to sensor 10.

The output voltage at the terminals 160 also drives 790 7407-13 also a negative feedback circuit 162, whereby the direct current flowing in the two-wire cable 14 j! is controlled. To this end, the output voltage is directed to a resistance network, generally indicated by 164.5 and also connected to the reference voltage V_, * X \ i and the -15 volt terminal, to provide translation j of the 0-10 volt output signal. to an area that is adapted to that of the 4-20 ma cable current. The resulting translated voltage is applied through a conductor 166 to one input of a voltage-to-current converter 168, which serves as a differential adder (see 80 in FIG. 2]. The other input to the converter is supplied with voltage, received via a ratio-impedance resistor network 17Q 15 (which includes a suitable filter, not shown) from the top end of the emitter resistor 52 of the constant current source 48. The output of the converter 168 drives a Darlington circuit connected pair of transistor 50 51 for controlling the line current 20 through resistor 52 so that it follows the output voltage at terminal 160. Thus, meter 20 supplies transmitter j 12 with a direct field indication of the actual value j of the final analog measurement signal developed j at the output terminal 160.

j 25 Regarding the specific circuit details | From the signal processors to the control room, it should be noted that the reference voltage VR is generated by a Zener diode 170, which is powered by the -15 volt terminal and serves to generate a reference voltage of about 9 volts.

Furthermore, a pair of series-connected diodes 172, 174 are connected across this Zener diode, with their common junction 176 connected! on the right end of a resistor connected to the I '{35 output of the first operational amplifier 130, in order to prevent the voltage at that point from entering the safe 7907407-14 region for switches S3 and S4 in the following j exceeds condensation switching circuit 134. Capacitors 178 and 180 are provided on both capacitor switching circuits to ensure filtering of the! * 5 high-frequency components, resulting from the! switching action. ,

Although in the above a preferred embodiment! liner of the invention has been described in detail, it should be emphasized that this is only for the purpose of illustrating the principles of the invention, and that the invention is not limited to this embodiment, as it is known to those skilled in the art it will be understood that modifications of the invention are possible without departing from its scope.

| ! - conclusions - 790 74 07

Claims (19)

1. Instrumentation system of the type containing a '{force sensor with a vibrating wire tensioned | in accordance with the input force, so that the resonant frequency of the wire is a measure of the applied force, characterized in that this system comprises the following combination: an oscillator coupled to the wire and thereby operative to develop an oscillator signal on the wire resonant frequency, 10 signal processing means comprising a frequency-to-analog converter means, a two-wire transmission cable coupled with one end to the oscillator and with the other end to the signal processing means, the transmission cable 15 for directing to the frequency-analog converter etter of a fast varying AC signal, which in frequency corresponds to the oscillator signal, whereby the converter means produces an analog measuring signal in response to the oscillator frequency, 2. DC signal means operable to produce in the transmission cable a DC signal corresponding to the analog signal. etching signal, and indicator means coupled to said one end of the transmission cable and adapted to produce a visual indication of the magnitude of the DC signal flowing through the transmission cable. 2. Device according to claim 1, characterized in that the sensor is located on a field post and the signal processing means are located on a central station, that the direct current signal has a "live zero", 790 7 4 07 ~ 16 ~ _ so that the current flows continuously through the two-wire cable at a level no less than a predetermined size, and that there are voltage supply means on the transmitter and operable with the DC signal to develop a unidirectional power supply voltage to provide operational power j to all elements that require power from the transmitter! 4 y 3. Device as claimed in claim 2, characterized in that the signal processing members further comprise a scale member for developing the analog measuring signal with determined zero and j span width characteristics. 1 i i 4. Device as claimed in claim 3, characterized in that the scale member develops the analog signal with a zero magnitude corresponding to applied zero force on the sensor. \ 5. An apparatus according to claim 3, characterized in that the signal processing means further comprise a linearizer for effecting a linear relationship between the analog signal and the applied force. 1 2 790 7 4 07 Device as claimed in claim 2, m e t h e t; C h a r a c e r e r i z e d in that the DC axis signal means 25 comprises a controllable constant current source. The device according to claim 6, characterized in that the constant current source comprises a transistor, connected in series with the two-wire cable and with an emitter resistor, and that the direct current signal means further comprises means for comparing the level of the voltage across the said resistor with the magnitude of the analog measurement signal, and for continuously adjusting the; potential difference between the base of the transistor! i and the remote end of the emitter resistor for providing a follow-up of the DC current with the analog measurement signal. 8. Apparatus according to claim 6, characterized in that the transmitter comprises means for applying an AC voltage signal to the two-wire cable, thunder thereby affecting the magnitude of the current flux by the constant current source. 9. Device as claimed in claim 8, characterized in that said means for applying the voltage signal comprise a Zener diode 1j in series with the two-wire cable, and | a switch, connected in parallel with the Zener diode and operated by the oscillator. 10. Instrumentation system for performing process condition measurements at one location and for transmitting corresponding signals to another location, characterized by a transmitter station, a state sensing element, which is part of the transmitter station, a signal generator at the transmitter station and coupled to the transmitter station. state sensing element for generating an alternating transmit signal whose frequency corresponds to said state, a signal receiving station, | a two-wire cable, which couples the transmitter station to the signal receiving station, a DC power supply to the 79074 07 * 18 signal receiver station and coupled to the two-wire cable to produce a DC through it, a DC circuit to the transmitter station, and! 5 coupled to the two-wire cable to develop a DC supply voltage of the DC current passing through the two-wire cable; means for coupling the DC power supply voltage to the signal generator to provide operational power thereto for generating the alternating transmission signal, means coupled to the two-wire cable for generating an alternating measurement signal in the cable with the frequency of said signal generator-13 signal, a constant current source at the signal reception; station and series connected to the two-wire cable for maintaining a constant cable current, which is not affected by said alternating voltage, and a frequency-analog converter means coupled to the two-wire cable at the signal receiving station and operative to produce an analog measurement signal, that corresponds to the frequency of the said alternating measuring signal in the cable. 11. Device according to claim 10, characterized in that it comprises a voltage regulating device: connected in series with the two-wire cable, and which normally produces a determined DC voltage thereon in response to the current flux through the; cable, and means coupled to the signal generator for bringing and maintaining the voltage across the voltage regulating device to a level less than "3" the set voltage synchronous with the output of 7907407 * - - 4. - 19 "signal generator, to provide an AC voltage to the two-wire cable. 12. Apparatus according to claim 10, characterized in that it comprises means for controlling the current level generated by the constant current source in accordance with a selected variable, and means at the transmitter station responsive to slow changes in the current level generated at the signal receiving station. 13. Instruction system for performing process state measurements at one location and for transmitting corresponding signals to another location, characterized by: a transmitter station, a state sensing element forming part of the transmitter station, | a signal generator at the transmitter station and t1 coupled to the state sensing element for producing a transmit signal of a frequency i corresponding to the state, a signal receiving station, | I a two-wire cable, which couples the transmitter station to the signal receiving station, DC power supplies to the signal receiving station, coupled to the two-wire cable to generate a current flux therethrough, which has an average DC component of limited size, 30 means for coupling the output of the signal generator with the two - wire cable for the | leading there an alternating measuring signal in accordance with the frequency of the signal generator output, 7907407 -20- a frequency-analog converter means coupled with the two-wire cable to the signal receiving station and operative to produce an analog measuring signal corresponding to the frequency of the 5 alternating measuring signal on the cable, controllable means at one of the stations, corresponding to a determined variable, direct current signal means coupled to the controllable means and with the two-wire cable to said one station, which direct current signal means are operative varying the magnitude of said average DC component in the cable ji according to the determined variable, and signal responsive means at the other station; 15 and coupled to the two-wire cable in order to conform to the magnitude of the average DC current! component as it is controlled by the DC signal means, so as to transmit information regarding the determined variable between the two stations, while simultaneously transmitting the alternating measurement signal from the transmitter station to the signal reception station. Device according to claim 13, characterized in that the direct current signal means 25 comprise a constant current source, which is connected in series with the two-wire cable, and that the controllable members are coupled to the constant current source in order to adjust the current level supplied thereby in the two-wire cable. . 15. Device according to claim 14, characterized in that the constant current source comprises a transistor in series with the two-wire cable, an emitter resistor connected to the transisitor, and 7907407-21 that the controllable means are operable to control the voltage between the base of the transistor and the remote end of the emitter resistor 16. Device as claimed in claim 14, characterized in that the controllable members comprise means responsive to said analog measuring signal for adjusting the current level through the cable in accordance therewith, and that there are means at the transmitter station, which respond to the current level set by the controllable members. 17. Device as claimed in claim 16, characterized in that the transmitter member comprises an indicator () which shows the current level. i ï | 18. Instrumentation system, comprising a process state scanner at a field station, which has an element with a characteristic that varies with the state being measured, characterized by | a transmitter at the field station, which includes a signal generator coupled to the sensor element for developing an alternating signal which responds | to the said characteristic, i! a central station, a two-wire transmission cable, which couples transmitter 25 to the central station, means for applying to a two-wire cable an alternating signal corresponding in frequency to the output of the signal generator, signal processing means at the central station, which include means responsive to the alternating signal and operative to generate a state measurement signal, scaled to correspond to a predetermined variation range of 790 7 407 4 K _ ~ 22 ~ _ the state,; . DC signal means operable to generate in the two-wire transmission cable a DC signal corresponding to the yellow scaled state measurement signal and indicator means to the field station and coupled! with the two-wire cable for the production of one; visual indication of the magnitude of the DC signal. i 19. Device as claimed in claim 18, characterized in that the signal processing means comprise means for adjusting the zero and span of the state measuring signal. 20. Device as claimed in claim 19, characterized in that the signal processing means comprise means for characterizing the 'state measuring signal in accordance with a determined transmission function. 21. Device according to claim 20, characterized in that the characterizing means comprise means for developing a linear relationship between the magnitude of the state measurement signal and said state. 790 7 4 07